Cancers contain altered methylation patterns that result in aberrant expression of critical genes. Hypermethylation turns off the expression of genes required to regulate normal growth, while hypomethylation allows the inappropriate expression of genes that permit cellular proliferation. Aberrant promoter hypermethylation occurs at the 5-prime position of cytosine within a CpG dinucleotide (Gardiner-Garden et al., J. Mol. Biol. 196(2): 261-82 (1987)). It inactivates the expression of critical genes that are involved in tumor suppression, DNA repair, control of tumor metastasis and invasion (Feinberg et al., Nature. 301: 89-92 (1983); Jones et al., Nat. Rev. Genet. 3(6): 415-28 (2002)). In colorectal cancer (CRC), for example, epigenetic silencing of O(6)-methylguanine-DNA methyltransferase is associated with G to A mutations in K-ras and p53 genes (Esteller et al., Cancer Research 61(12):4689-92 (2001); Esteller et al., Cancer Research 60(9):2368-71 (2001)). Hypermethylation of the mismatch repair gene, hMLH1, is linked to a sporadic microsatellite instability phenotype in colon tumors (Herman et al., Proc Natl Acad Sci USA. 95(12):6870-6875 (1998); Kane et al., Cancer Res. 57(5):808-811 (1997)). Furthermore, the hypermethylated p16INK4a and p14ARF reside in a genomic region (9p21) that commonly undergoes loss of heterozygosity, suggesting that methylation silencing may cooperate with other genetic alterations for gene inactivation (Weber et al., Cytogenet Cell Genet. 86(2):142-147 (1999)).
Retinoids, a class of natural and synthetic vitamin A analogues, are important therapeutic agents used in oncology and hematology (Altucci et al., Nat Rev Cancer 1(3):181-193 (2001); Niles R. M., Mutat Res. 555(1-2):81-96 (2004)). Retinoids are metabolized into two main classes of biologically active compounds, retinal and retinoic acid (RA). Retinal is essential for the formation of rhodopsin the visual chromophore, while RA serves as an important factor in regulating the expression of a large number of genes, primarily by functioning as a ligand activator for two families of nuclear retinoid receptors: retinoic acid receptors (RARs) and retinoid X receptors (RXRs) (Altucci et al., Trends Endocrinol Metab. 12(10):460-468 (2001); Kastner et al., Development. 124(2):313-326 (1997); Mangelsdorf et al., Cell 83(6):835-839 (1995)). Adequacy of vitamin A and its metabolites have been linked to the occurrence of various human cancers (Crowe et al., Mol Cancer Res. 1(7):532-540 (2003); Hayden et al., Breast Cancer Res Treat. 72(2):95-105 (2002); Mahmoud et al., Int J Cancer. 30(2):143-145 (1982)). In CRC, aberrant crypt foci (ACF) are proposed to be preneoplastic lesions occurring in hyperproliferative human colon tissues and carcinogen-treated laboratory animals. The formation of carcinogen-induced ACF can be inhibited by retinol, 9-cis-RA, and 4-(hydroxyphenyl)retinamide in animal models (Wargovich et al., Carcinogenesis. 21(6):1149-1155 (2000); Zheng et al., Carcinogenesis 20(2):255-260 (1999)). In vitro matrigel and in vivo xenograft models of CRC treated with trans-RA, 9-cis-RA and 13-cis-RA show reduced MMP7 expression and proteolytic degradation of the extracellular matrix, important mechanisms of tumor invasion (Adachi et al., Tumour Biol. 22(4):247-253 (2002)). In addition, several in vitro studies indicate that retinoids have potent antiproliferative effects on CRC cell lines and may have chemopreventive and chemotherapeutic potential for CRC (Briviba et al. Biol Chem. 382(12):1663-1668 (2001); Callari et al., Int J Oncol. 23(1):181-188 (2003); Park et al., Cancer Res. 65(21):9923-9933 (2005)). The association between retinoid levels and cancer development suggests that retinoids offer great promise for cancer therapies and most studies have focused on the retinoid signaling pathways in suppressing carcinogenesis. Although the key players of retinoid biosynthesis have been identified, the mechanism of regulating the cellular RA concentration is not well understood, but is critically related to tumor development.
Retinoids are metabolized via sequential oxidation steps (shown in FIG. 1). The key molecules involved in the metabolism consist of a family of retinol dehydrogenases (RDHs), several class I aldehyde (retinal) dehydrogenases (ALDHs/RALDHs), a family of chaperone-like regulatory proteins the cellular retinol-binding proteins (CRBPs) and the cellular retinoic acid binding proteins (CRABPs) (Sophos et al., Chem Biol Interact. 143-144:5-22 (2003); Yoshida et al., Eur J Biochem. 251(3):549-557 (1998); Wei et al., Dev Dyn. 201(1):1-10 (1994); Ong et al., Biochim Biophys Acta. 1482(1-2):209-217 (2000); Vogel et al., J Biol Chem. 276(2):1353-1360 (2001)). Furthermore, a plausible mechanism has been proposed that two “gate-keeping” molecules, lecithin:retinol acyl transferase (LRAT) and cytochrome P450 enzymes (CYP26s), are coordinately regulated by all-trans RA to control the availability of retinol and RA, respectively (Ross, J Nutr. 133(1):291S-296S (2003)). The synthesis of retinyl esters (RE), the principal cellular storage form of retinol in the stellate cells of the liver, is catalyzed by LRAT and a less characterized enzyme acyl CoA:retinol acyltransferase (ARAT) (Guo et al., Carcinogenesis 21(11):1925-1933 (2000)). Genetic studies have shown that CYP26s convert RA to less active and more readily excretable polar metabolites. A deficiency of retinoic acid biosynthetic enzymes in colon epithelial tissues has been proposed to lead to decreased mucus production, expansion of proliferation zones within the crypt, ion flux alterations, and development of premalignant and malignant cells during tumorigenesis (Jette et al., J Biol Chem. 279(33):34397-34405 (2004)). Understanding these regulating factors may facilitate the use of dietary or pharmacological means for the prevention and improved treatment of human cancer.
The present invention is directed to overcoming these and other deficiencies in the art.